When transmitting high speed signals across a PCB (e.g., multi-gigabit per second signals), the signals are typically routed as differential signals. In differential signaling, two complimentary signals are transmitted along two conductive paths, forming a differential pair. The signals are typically at opposite reference voltages such that as one signal voltage transitions from state 1 to state 2, the other signal voltage transitions from state 2 to state 1. Differential signals are less sensitive to various types of noise present in a PCB because a difference between the two signals is used to encode the information instead of an absolute voltage of the signals. Often, differential signals are routed between different signal layers on a PCB. A PCB may have signal layers on the outer layers where electronic components are mounted, and on inner layers of the PCB. For example, a 10 layer PCB has 2 outer layers and has 8 inner layers. When a PCB designer elects to route signals from one layer to another layer (e.g., from a first signal layer to a second signal layer), a “via” is used. A via is a conductive PCB structure that spans the transition space between the two signal layers. When routing a differential signal through a pair of vias, the two vias are referred to as differential vias. When forming a via on a PCB, a drilling process is first used to generate a hole between the two layers. A deposition process is then performed (e.g., plating) within the hole to form a conductive path (i.e., a barrel) between the first signal layer and the second signal layer. Pads are then fabricated on each end of the via barrel spanning the two signal layers. The pads allow the signals to transition between the layers of the PCB. In some cases, the via will span a conductive plane within the PCB. For example, when the via spans a layer used as a ground plane or a power plane, the plane will include a non-conductive spacing between the plane and the barrel of the via. Without the spacing, the via would electrically short to the plane. This spacing is called the anti-pad for the via, and is part of the design process used to define the via.
In order to save cost, PCB fabricators will often drill a hole through the PCB from one component layer to the other, and plate the hole. In this case, the signal layers being routed may be deep within the PCB on the inner layers, while the ends of the via near the component layers may be unconnected. The unconnected ends of the via are called “stubs,” because they typically protrude beyond the desired signal path. Stubs are problematic for PCB designers when high speed signals are routed along the via because the stubs may couple electrically to components mounted to the component layers of the PCB or other vias proximate to the stubs. In order to reduce the coupling, PCB fabricators may drill into the PCB along the via to remove the stubs. However, this increases the fabrication costs and may also damage the via internally when the drill penetrates too far into the PCB.
When high speed differential signals are routed on conductive traces of the PCB, the conductive traces act like differential transmission lines. In this case, PCB fabricators and designers take care in designing the traces to have a characteristic differential impedance that remains constant across the PCB. A typical value of the characteristic differential impedance is one hundred ohms, for example. This impedance control may be accomplished by controlling a width of the conductive traces and a distance between the two conductive traces. It remains important that the characteristic differential impedance remains relatively the same when the differential signal encounters the differential vias. If the input dynamic differential impedance of the differential vias varies with respect to the characteristic differential impedance of the conductive traces at high frequencies, then the differential signals are distorted as they traverse the differential vias, resulting in an output differential signal from the two output vias that is different from the input differential signal to the two input vias. Therefore, it is important to understand how vias affect the signals they carry in the PCB.